Myopathic Syndromes

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In general, diseases of muscle (myopathies) are recognized by the characteristic pattern of proximal shoulder and hip girdle weakness with relative preservation of distal strength (see Table 15-2 ). Myopathic syndromes also differ from symmetrical polyneuropathies in their effect on reflexes. In contrast to polyneuropathies, in which the distal reflexes are lost early, in myopathies the reflexes are reduced in proportion to the degree of weakness; only with severe weakness in the end stages of a myopathy are the reflexes lost (see T.a.ble...J..5-5). Myopathies can be further differentiated from neuropathies by the lack of any sensory abnormalities. Atrophy of muscle may occur in myopathies but develops very slowly over the course of several years, whereas in motor neuron disease it develops much more rapidly. Also, no fasciculations are seen in myopathies. In certain muscular dystrophies, especially Duchenne's muscular dystrophy, pseudohypertrophy of the calf and deltoid muscles may occur owing to infiltration of the muscle by fat. The patient with myopathy often has a typical facial expression with ptosis, open mouth, and facial immobility.

As is often the case in medicine, there are exceptions to these general rules. The distal muscles may be weaker than the proximal muscles (e.g., myotonic dystrophy, inclusion body myositis), or a motor neuron disorder may affect the proximal musculature more than the distal, mimicking a myopathy (Kugelberg-Welander syndrome). Some myopathies affect the cranial and ocular muscles rather than limb muscles, as in, for example, the chronic progressive external ophthalmoplegia (CPEO) syndrome. The abnormal eye movements can be differentiated from those in myasthenia gravis; in CPEO eye movements are restricted symmetrically and diplopia is not a complaint, whereas in myasthenia gravis double vision is a common symptom.

If special attention is given to the distribution of muscle weakness, the age and sex of the patient, and the family history, the likely diagnosis can often be determined. Some myopathies are congenital and have a different prognosis from rapidly progressive myopathies of childhood onset

TABLE 15-14 -- "ACUTE" GENERALIZED WEAKNESS

Location of Lesion

Differentiating Features

Causes

Midbrain

Coma, mid-size poorly reactive pupils, decorticate/decerebrate posturing, hyperreflexia, bilateral Babinski's signs

Stroke, trauma, tentorial herniation secondary to mass lesion

Basis pontis

Conscious; paralysis of horizontal eye movements and of jaw, face, pharynx, and tongue with preservation of vertical eye movements, eye blink; UMN spastic tetraparesis ("locked-in" syndrome)

Stroke

Spinal cord (spinal shock)

Flaccid paraplegia (tetraplegia), sensory loss below level of lesion with absent tendon reflexes but with extensor plantar responses; hypotonic bladder and sphincters

Trauma, infarction, metastatic tumor, transverse myelitis

Polyradicular neuropathy

Limb weakness (legs before arms), areflexia, absent or flexor toe response, minimal distal sensory loss without a sensory level

Acute inflammatory demyelinating polyneuropathy, tick paralysis, poliomyelitis

Neuromuscular junction

Ocular (including loss of pupillary light reflex) and pharyngeal weakness followed by generalized weakness without sensory loss and preservation of consciousness

Botulism, organophosphates

in boys that are usually dystrophinopathies. Dominantly inherited dystrophies have a later onset and a more benign course and have a rather uniform pattern of involvement (e.g., fascioscapulohumeral dystrophy). Mitochondrial myopathies preferentially affect the ocular and cranial musculature and are often accompanied by dysfunction in other organ systems. Inflammatory and infectious myopathies generally affect older individuals. The exception is dermatomyositis, which may occur at any age but is accompanied by characteristic skin changes.

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